Polyisocyanates, such as aliphatic and aromatic isocyanates like diisocyanates, have been polymerized in the presence of a trimerization catalyst to provide rigid polyisocyanurate foams. The polyisocyanate trimerization reaction has been carried out in bulk and in solution to provide essentially cross-linked, very brittle and very friable foam products. The foam products are produced by combining polyisocyanate, an inert blowing agent, such as a low-boiling-point liquid like halocarbons, and one or more trimer catalysts like a tertiary amine, and mixing to effect the polyisocyanurate reaction. Typically, the reaction is exothermic on mixing the components, and no additional heating is required. The polyisocyanurate foams produced to date have not been commercially acceptable, and have been characterized by high brittleness and friability. Brittleness refers to the internal friability of the foam structure which remains essentially unchanged with time; that is, it is structural and molecular in nature, while friability refers to the state of the surface of the polyisocyanurate foam; that is, the powderability of the surface when subject to pressure, which friability changes with time.
Attempts to reduce the friability of polyisocyanurates have been made by modification of the polyisocyanurate principally through the introduction of other chemical linkages. Epoxy-modified isocyanurates have not been commercially acceptable, since they are expensive, the reaction is difficult to control and the materials exhibit limited processing properties. Imide-modified isocyanurates are very thermally stable with high thermal conductivity, but also are prepared from expensive raw materials. Carbodiimide-modified isocyanurates also are expensive and the reaction is difficult to control. The present-day, best, commercial products are the urethane-modified isocyanurate products which, although expensive, can be prepared by known and readily available commercial catalytic agents, and provide foams of relatively good thermal and flame-resistant properties.
In the preparation of such polyisocyanurate; i.e., trimer, foams consisting essentially of recurring cross-linked isocyanurate units and the modified polyisocyanurates, a wide variety of trimer catalysts and combinations have been suggested and used (see, for example, U.S. Pat. Nos. 3,487,080; 3,723,364; 3,736,298; and 3,759,916). Such trimer catalysts have included tertiary amines, such as N,N'-dialkylaminoalkyl phenols and the like.
The base trimerization of isocyanates in the presence of ethylene carbonate has been reported to result in the acceleration of the trimerization process and in the formation of a solid complex of the polyisocyanurate-ethylene carbonate (see Tsuzuki et al, "New Reactions of Organic Isocyanates. I. Reaction with Alkylene Carbonates," Journal of Organic Chemistry, Vol. 25, 1009, June 1960). Further, the reaction of propylene carbonates with polyisocyanurates is set forth in Saunders and Frisch, "Polyurethanes: Chemistry and Technology," High Polymers, Vol. XVI, Part 1, page 116, Interscience Publishing Co., Inc. Organic carbonates have also been used as modifiers in noncellular urethane resins (see U.S. Pat. No. 3,883,466).
It is, therefore, most desirable to provide trimer isocyanurate foams, both modified and unmodified, which have low friability while retaining the other desirable properties of such foams, and methods of preparing such foams which provide for improved and rapid process conditions and cure of the polyisocyanurate foams.